1. Field of the Invention
The present invention relates to an MEMS sensor and a production method thereof.
2. Description of Related Art
In recent years, an MEMS sensor such as an Si (silicon) microphone produced by MEMS (Micro ElectroMechanical Systems) has been employed as a microphone loaded on a portable telephone or the like.
FIGS. 3A to 3I are schematic sectional views successively showing the steps of producing a conventional Si microphone 101. The method of producing the conventional Si microphone 101 and the structure thereof are now described with reference to FIGS. 3A to 3I.
In order to produce the conventional Si microphone 101, SiO2 (silicon oxide) films 111 (111A and 111B formed on the upper and lower surfaces of an Si wafer W2 respectively) are formed on the overall surfaces of the Si wafer W2 by thermal oxidation, as shown in FIG. 3A.
Then, a plurality of (four in FIG. 3B) recesses 112 are formed in the upper surface of the SiO2 film 111A by well-known photolithography and etching, as shown in FIG. 3B.
Then, polysilicon is deposited on the overall surfaces of the SiO2 films 111 by LPCVD (Low Pressure Chemical Vapor Deposition). The polysilicon film covering the SiO2 film 111A is doped with phosphorus, and portions of this polysilicon film other than that present on a prescribed region including the plurality of recesses 112 are thereafter removed by well-known photolithography and etching. Thus, a thin-film polysilicon plate 104 is formed on the prescribed region of the SiO2 film 111A, as shown in FIG. 3C. Further, a polysilicon film 113 is formed on the SiO2 film 111B.
Then, SiO2 is deposited on the overall surfaces of the SiO2 film 111A and the polysilicon plate 104 by PECVD (Plasma Enhanced Chemical Vapor Deposition). Then, unnecessary portions of the deposited SiO2 film are removed by well-known photolithography and etching. Thus, a sacrificial layer 114 is formed on the polysilicon plate 104 and a region around the same, as shown in FIG. 3D.
Then, polysilicon is deposited on the SiO2 film 111A, the sacrificial layer 114 and the polysilicon film 113 by LPCVD (Low Pressure Chemical Vapor Deposition). Thus, the polysilicon film deposited on the polysilicon film 113 and the polysilicon film 113 are integrated into a polysilicon film 115, as shown in FIG. 3E. On the other hand, the polysilicon film deposited on the SiO2 film 111A and the sacrificial layer 114 is doped with phosphorus, and thereafter patterned by well-known photolithography and etching. Thus, a thin-film back plate 105 having a large number of holes 106 is formed on the sacrificial layer 114, as shown in FIG. 3E.
Then, a plurality of (four in FIG. 3F) recesses 117 are formed in the upper surface of the sacrificial layer 114 by well-known photolithography and etching, as shown in FIG. 3F. Further, unnecessary portions (other than that opposed to the sacrificial layer 114) of the SiO2 film 111A are removed.
Then, an SiN (silicon nitride) film 107 is formed by PECVD to cover the sacrificial layer 114, as shown in FIG. 3G.
Then, holes 118 communicating with the holes 106 of the back plate 105 are formed in the SiN film 107 by well-known photolithography and etching, as shown in FIG. 3H. Thus, the sacrificial layer 114 is partially exposed through the holes 106 and 118. Further, an opening is formed in a portion of the SiO2 film 111B opposed to the polysilicon plate 104 by well-known photolithography and etching. The Si wafer W2 is so etched through this opening that a through-hole 103 is formed therein. Consequently, the SiO2 film 111A is partially exposed through the through-hole 103.
Then, an etching solution capable of etching SiO2 is supplied through the through-hole 103 and the holes 106 and 118, to wet-etch the sacrificial layer 114 and the SiO2 film 111A. Thus, the polysilicon plate 104 floats up from the upper surface of the Si wafer W2 while a cavity 110 of a small interval is formed between the polysilicon plate 104 and the back plate 105, as shown in FIG. 3I.
Thereafter the Si wafer W2 is diced into an Si substrate 102 of each device size, whereby the Si microphone 101 is obtained with the polysilicon plate 104 and the back plate 105 opposed to each other through the cavity 110. Portions of the SiN film 107 having entered the recesses 117 of the sacrificial layer 114 become protrusions 109 protruding toward the polysilicon plate 104, to function as stoppers for preventing the polysilicon plate 104 and the back plate 105 from adhesion and a short circuit. Further, portions of the polysilicon plate 104 having entered the recesses 112 of the SiO2 film 111A become protrusions 108 protruding toward the upper surface of the Si wafer W2, to function as stoppers for preventing the Si substrate 102 and the polysilicon plate 104 from adhesion. The polysilicon plate 104 and the back plate 105 are supported by unshown wires.
In this Si microphone 101, the polysilicon plate 104 and the back plate 105 covered with the SiN film 107 form a capacitor portion 120 opposed through the cavity 110. When a sound pressure (sound wave) is input in the Si microphone 101 from above the back plate 105, the back plate 105 and the polysilicon plate 104 vibrate due to this sound pressure, and the capacitor portion 120 outputs an electric signal responsive to a change of the capacitance of the capacitor portion 120 resulting from this vibration of these plates 104 and 105.
In the Si wafer W2, the capacitor portion 120 is formed by the thin-film polysilicon plate 104 and the thin-film back plate 105. Therefore, the capacitor portion 120 may be deformed or broken by coming into contact with another substance.
When water for removing frictional heat (cooling) is supplied to a dicing saw in a dicing step, for example, the water hits the capacitor portion 120, to deform or break the capacitor portion 120 by this shock. If a dicing tape is bonded to the capacitor portion 120, the capacitor portion 120 is broken when the dicing tape is separated therefrom. Therefore, neither the dicing saw nor the dicing tape can be employed, but a specific technique such as laser dicing must be employed for dicing the Si wafer W2.
When the Si microphone 101 is carried or a system employing the Si microphone 101 is assembled after the Si wafer W2 is diced into each device size, a semiconductor device or the like mixedly provided on the system may come into contact with the capacitor portion 120, to deform or break the capacitor portion 120.